There is a need for an electro-optical device that in one or more light states is transparent to visible light and in other light states attenuates light. Applications for such a device include use as a light attenuator in a smart glass, use as a see-through display, or use as a sunlight-readable, reflective display. By controlling light transmittance in windows, glass facades or roof systems, functions including see-through (i.e. transparent), privacy (opaque), electronically-variable tinting or dimming, or black-out can be provided. In display devices functionality can be extended into new areas such as providing see-through (i.e. transparent) displays and providing large-format, sunlight-readable, reflective displays for outdoor applications.
There is a need for an electrophoretic device to operate over the wide temperature range encountered outdoors in a geographic location (i.e. a local areas' climate). An insulated glass unit (IGU) that incorporates a laminated electrophoretic device in its outer pane exposes the device to approximately outdoor temperatures. Furthermore, an IGU incorporated device is exposed to sunlight and requires the necessary photostability (i.e., weatherability).
At ground level in hot climates the energy level in the three sunlight spectrums per square meter with the sun at its zenith is about 32 watts (3%) of UV, 445 watts (44%) of visible, and 527 watts (53%) of infrared, or about 1,004 watts total. In general, the prior art of smart glass and outdoor displays does not describe what happens this solar energy when it is absorbed by light states, and is largely silent with respect to operating at elevated temperature due to heat build-up within a device that absorbs incident solar energy.
The problems an electrophoretic device encounters due to use at elevated temperature include significantly increased haze with respect to a normative (i.e. reference) ambient temperature, and significantly increased ability of the suspending fluid to dissolve/solubilize or swell polymer elements within the electrophoretic cell and in contact with the fluid (e.g., non-planar polymer structures and charged particles). The haze problem is proportional to temperature and reversible. An electrophoretic device having minimal haze (i.e. about 3% or less) at 20 degrees C. (the refractive index reference temperature) can develop a perceivable increase in haze from about 30 degrees C. and go on to become cloudy or translucent at about 70 degrees C. The polymer dissolution or swelling problem develops over time and using prior art devices/methods results in device failure over days or months depending on material selection and the total number of hours at elevated temperature. For example, in devices that use aliphatic hydrocarbons for the suspending fluid (e.g., the Isopar range from Exxon Mobil) the suspending fluid can become an effective solvent for a polymer at elevated temperature while being a non-solvent at room temperature. Furthermore, unless measures are taken to manage heat build-up in very hot climates the maximum continuous use temperature of a polymer can be exceeded leading to thermal degradation over time. For example, PMMA (i.e. polymethylmethacrylate) and most polyurethanes have a maximum continuous use temperature (also known as its service temperature) of about 90 degrees C. or less.
Very low operating temperature also causes significantly increased haze with respect to a normative operating temperature, and the suspending fluid viscosity can increase to multiples of its value at 20 degrees C. leading to unacceptably long switching times or failure to switch. In prior art devices that use linear aliphatic hydrocarbons for the suspending fluid, wax crystals can form in the fluid causing a cloudy or translucent appearance from about −10 degrees C. or lower.
In summary, there is a need for an electrophoretic device to remain haze-free (i.e. minimal haze) and operate reliably over the wide temperature range encountered outdoors in geographic areas intended for use. Furthermore, due to solar energy absorption in some light states, elevated temperature operation significantly higher than peak ambient temperature is required. The principle problems associated with prior art devices when operated over a wide temperature range and exposed to sunlight can include some or all of the following:                a) haze that increases in proportion to the temperature difference with a normative or reference temperature, and becomes perceivable from about 10 degrees Celsius difference or more;        b) polymer dissolution or swelling by the suspending fluid at elevated temperature over time;        c) thermal degradation over time if a polymer is subject to continuous use greater than its service temperature;        d) unacceptable long switching times or failure to switch at low ambient temperature;        e) suspending fluid crystallization at low ambient temperature; and,        f) sunlight induced photo degradation of polymer and suspending fluid materials over time.        